The present invention relates generally to integrated gasification combined-cycle (IGCC) power generation plants, and more particularly, to methods and apparatus for optimizing substitute natural gas production and heat transfer with a gasification system.
At least some known IGCC plants include a gasification system that is integrated with at least one power-producing turbine system. For example, known gasification systems convert a mixture of fuel, air or oxygen, steam, and/or carbon dioxide (CO2) into a synthesis gas, or “syngas”. The syngas is channeled to the combustor of a gas turbine engine, which powers a generator that supplies electrical power to a power grid. Exhaust from at least some known gas turbine engines is supplied to a heat recovery steam generator (HRSG) that generates steam for driving a steam turbine. Power generated by the steam turbine also drives an electrical generator that provides electrical power to the power grid.
At least some known gasification systems associated with IGCC plants include a gasification reactor that produces a syngas that includes at least some carbon monoxide (CO), water vapor (H2O) and particulate matter. The syngas from the gasification reactor is channeled to a scrubbing and quenching assembly that typically removes a substantial portion of the particulate matter and cools the syngas by injecting water into the syngas. Subsequently, in order to increase the amount of combustibles within the syngas, the scrubbed and quenched syngas is typically channeled to at least one water-gas shift reactor to convert the CO and water into hydrogen (H2) and carbon dioxide (CO2) via at least one exothermic chemical reaction. The heat released via the exothermic reactions facilitates a temperature rise in the shift reactor.
At high temperatures and low water content, the CO may react with the H2 to produce methane (CH4) and CO2 via an exothermic chemical reaction. At temperatures in excess of approximately 650 degrees Celsius (° C.) (1200 degrees Fahrenheit (° F.)), the rate of CH4 and CO2 production reactions may accelerate such that control of the heat release within the shift reactor is reduced. Therefore, steam is often injected to mitigate the temperature rise and facilitate control of the temperature within the shift reactor. The amount of steam used typically amounts to approximately 30% to 50% of the high pressure steam that could otherwise be channeled to the steam turbine, thereby reducing the plant's electrical generation. Moreover, such steam injection requires a larger shift reactor to produce an adequate supply of shifted syngas. Furthermore, the increased moisture content of the syngas channeled from the shift reactor forms a need for additional moisture removal apparatus.